In the printer’s manuscript, Oliver Cowdery initially wrote simply “the master”, then corrected the text by inserting “of the vineyard”. The level of ink flow is the same, which suggests that his correction here in 𝓟 is an almost immediate one and is based on the reading of the original manuscript, no longer extant here.
Later, the 1837 edition omitted the phrase once more, probably by accident. There is nothing wrong with “the master of the vineyard”, nor was it marked for deletion by Joseph Smith in the printer’s manuscript. In fact, this phrase occurs two other times in the olive tree allegory and without variation:
The phrase “the master” (that is, without “of the vineyard”) occurs nowhere else in the allegory. There are four occurrences of “his master” (verses 16, 21, 34, and 48), where of course it would be inappropriate to add “of the vineyard”.
Summary: Restore the whole phrase “the master of the vineyard” to Jacob 5:20, the expression that is consistent with the reading of the printer’s manuscript as well as with the two other occurrences of this phrase in the allegory.